DEVELOPMENTAL STAGES 



95 



lum that the animal moves through the blood or other fluids of 

 the body, either forward or backwards, so rapidly that it is difficult 

 to observe under the high power of a microscope as it wends its 

 way between the blood corpuscles on a slide. The body of the 

 animal contains, in addition to the large round nucleus near the 

 middle, another deeply-staining structure, the parabasal body 

 (see p. 31) at the posterior end near where the flagellum origi- 

 nates. The body also contains other granules of various sizes. 



There are a great many kinds of trypanosomes inhabiting many 

 different animals. Those living in cold-blooded animals have 

 no apparent effect on their hosts but the species infesting mammals 

 almost always cause disease. In man their effect is particularly 

 deadly and the African species usually cause death if allowed to 

 run to the sleeping sickness stage. Unlike many kinds of para- 

 sites most trypanosomes can live in a great many different hosts. 

 The common sleeping sickness trypanosome, for instance, can 

 live not only in man but also in monkeys, dogs, rodents, domestic 

 animals and a large number of the wild game animals of Africa. 



Most kinds of trypanosomes, like the malarial parasites, live 

 only part of their life histories in the blood or other fluids of 

 their vertebrate hosts, undergo- 

 ing another phase of it in the 

 digestive tracts of insects or other 

 invertebrates. In their interme- 

 diate hosts they undergo remark- 

 able transformations; the whole 

 series of forms through which 

 trypanosomes may pass in their 

 development, and which may 

 represent a phylogenetic as well 

 as an ontogenetic series, is shown 

 in Fig. 18. The first or Leish- 



. f i- r j .v FIG. 18. Diagram of developmental 



mama form, which Stands at the type sof trypanosomes; A, trypanosome 



foot of the Series, is a rounded form; B, Crithidial form; C, Herpeto- 



, , . . , monas form ; D, Leishmania form. (After 



body with a large central nu- wenyon.) 

 cleus and small rod-shaped para- 

 basal body usually set at a tangent to the nucleus (Fig. 18D). 

 Next in development comes the Herpetomonas form which differs 

 in having a long slender body and in having a flagellum produced 

 from the parabasal body (Fig. 18C). Next comes the Crithidia 



